Secondary Uses for the Lucas AVC Graphics Card

by Stan. Gent

The Lucas Advanced Video Controller provides an easily used, moderately high-definition colour graphics facility on the Nascom range of computers.

However, I have seen no mention of the fact that it can also provide a very useful addition to the memory capacity of the machine in non-graphical applications, by virtue of the fact that the board contains 48 kilo-bytes of dynamic RAM, arranged in 3 pages of 16K each.

These three pages are all located at the same address (normally 8000H to BFFFH, although there is provision for changing this by means of hard-wired links). Page selection is via port B2, but only operates after execution of the AVC control software (G32 or G48). As the execution of G32 or G48 seems to require the prior initialisation of Basic, even if all subsequent work is to be in machine code, it is also a wise precaution to set the Basic memory ceiling below 8000H. The response “32767” or less to the question “Memory size?” will achieve this.

Using Sys, the keyboard command

O B2 1

will select page 1 (normally the red page),

O B2 2

will select page 2 (green) and

O B2 3

will select page 3 (blue), all of which are 16K long.

If the Nascom also has its normal RAM extending over the sane address area, O B2 0 will select the normal RAM, giving, in effect, a choice of 4 separate pages of 16K each, in the location 8000H to BFFFH.

Obviously, the page selection can also be carried out within a program, and it is here that the full benefit can be realised. Listing 1 shows one way of achieving this in machine code whilst, in Basic, only a single-line instruction is required as in listing 2. (If using Basic, the memory ceiling limitation referred to earlier should not be applied.)

Don’t forget, of course, that if your control program is in the area of memory that you have just paged out, even a Nascom